Circulating Conjugated and Unconjugated Vitamin D Metabolite Measurements by Liquid Chromatography Mass Spectrometry

CONTEXT

Vitamin D status is conventionally defined by measurement of unconjugated circulating 25-hydroxyvitamin D (25OHD), but it remains uncertain whether this isolated analysis gives sufficient weight to vitamin D's diverse metabolic pathways and bioactivity. Emerging evidence has shown that phase II endocrine metabolites are important excretory or storage forms; however, the clinical significance of circulating phase II vitamin D metabolites remains uncertain.

OBJECTIVE

In this study we analyzed the contribution of sulfate and glucuronide vitamin D metabolites relative to unconjugated levels in human serum.

METHODS

An optimized enzyme hydrolysis method using recombinant arylsulfatase (Pseudomonas aeruginosa) and beta-glucuronidase (Escherichia coli) was combined with liquid chromatography mass spectrometry (LC-MS/MS) analysis to measure conjugated and unconjugated vitamin D metabolites 25OHD3, 25OHD2, 3-epi-25OHD3, and 24,25(OH)2D3. The method was applied to the analysis of 170 human serum samples from community-dwelling men aged over 70 years, categorized by vitamin D supplementation status, to evaluate the proportions of each conjugated and unconjugated fraction.

RESULTS

As a proportion of total circulating vitamin D metabolites, sulfate conjugates (ranging between 18% and 53%) were a higher proportion than glucuronide conjugates (ranging between 2.7% and 11%). The proportion of conjugated 25OHD3 (48 ± 9%) was higher than 25OHD2 conjugates (29.1 ± 10%) across all supplementation groups. Conjugated metabolites correlated with their unconjugated forms for all 4 vitamin D metabolites (r = 0.85 to 0.97).

CONCLUSION

Sulfated conjugates form a high proportion of circulating vitamin D metabolites, whereas glucuronide conjugates constitute a smaller fraction. Our findings principally in older men highlight the differences in abundance between metabolites and suggest a combination of both conjugated and unconjugated measurements may provide a more accurate assessment of vitamin D status.

Multiplex LC-MS/MS for simultaneous determination of 25-hydroxyvitamin D, 24,25-dihydroxyvitamin D3, albumin, and vitamin D-binding protein with its isoforms: One-step estimation of bioavailable vitamin D and vitamin D metabolite ratio

Bioavailable vitamin D and vitamin D metabolite ratio (VMR) have emerged as potential novel vitamin D markers. We developed a multiplex liquid chromatography-tandem mass spectrometry (LC-MS/MS) method to determine all elements necessary for the calculation of bioavailable vitamin D and VMR, including 25-hydroxyvitamin D [25-(OH)D] and 24,25-dihydroxyvitamin D₃ [24,25-(OH)₂D₃], VDBP and its isoforms, and albumin. Following separate reactions of hexane extraction and trypsin digestion, serum samples were analyzed using LC-MS/MS to measure 25-(OH)D₃, 25-(OH)D₂, 24,25-(OH)₂D₃, VDBP and its isoforms, and albumin. Analytical performances were assessed. Korean (n = 229), Arab (n = 98), White (n = 99) and Black American (n = 99) samples were analyzed. Bioavailable vitamin D and VMR were calculated. All target molecules were clearly separated and accurately quantified by LC-MS/MS. Analytical performances, including imprecision, accuracy, ion suppression, limit of quantification, linearity, and comparison with existing methods were within acceptable levels. The allele frequencies of VDBP isoforms in various races resulted similar to previously known values. The levels of bioavailable vitamin D were highest in White Americans and lowest in Black Americans. We have successfully developed a multiplex LC-MS/MS-based assay method that can simultaneously perform the measurement of all parameters needed to calculate bioavailable vitamin D and VMR. Our devised method was robust and reliable in terms of analytical performances and could be applied to routine clinical samples in the future to more accurately assess vitamin D status.

Development and application of a LC-MS/MS assay for simultaneous analysis of 25-hydroxyvitamin-D and 3-epi-25-hydroxyvitamin-D metabolites in canine serum

Hypovitaminosis D and hypervitaminosis D are well recognised disorders in dogs. Hypovitaminosis D can occur following consumption of a diet inadequately supplemented with vitamin D or as a sequelae of severe intestinal disease. Hypervitaminosis D may occur as a result of consuming proprietary dog foods over-supplemented with vitamin D or through ingestion of vitamin D containing medicinal products or rodenticides. Consequently, there is a clear need to establish a methodology that can accurately quantify vitamin D metabolites across a broad dynamic range in dogs. The existence of C3-epimers of vitamin D metabolites has yet to be elucidated in dogs, yet are known to interfere with the analysis of vitamin D and have unknown biological activity in other species. Here, we describe the development and validation of a sensitive, specific and robust analytical liquid chromatography tandem mass spectrometry (LC-MS/MS) assay capable of separating and accurately measuring 25-hydroxyvitamin-D_2/3 (25(OH)D_2/3) and 3-epi-25-hydroxyvitamin-D_2/3 (3-epi-25(OH)D_2/3). We describe a simplified workflow utilising supported liquid extraction (SLE) without derivatization that provides good linearity (mean r > 0.996) and accuracy across a broad dynamic range of 4-500 nmol/L for D₃ metabolites and 7.8-500 nmol/L for D₂ metabolites. Upon application of this assay to 117 canine serum samples, 25(OH)D₃ was detectable in all samples with a median concentration of 82.1 nmol/L (inter-quartile range (IQR) 59.7-101.8 nmol/L). 3-epi-25(OH)D₃ could be detected in 87.2 % of the study population, with a median concentration of 5.2 nmol/L (2.4-8.1 nmol/L). However, 3-epi-25(OH)D₃ was quantified below the LLOQ in 40.2 % of these samples. 3-epi-25(OH)D₃ contributed on average 6.3 % to 25(OH)D₃ status (contribution ranges from 0 to 23.8%) and a positive correlation was detected between 25(OH)D₃ and 3-epi-25(OH)D₃ concentrations. Free 25(OH)D was also measured using an immunoassay with a median concentration of 15.2 pmol/L (12.5-23.2 pmol/L), and this metabolite was also positively correlated to both 3-epi-25(OH)D₃ and 25(OH)D₃ concentrations. D₂ metabolites were not detected in canine serum as expected. Vitamin D metabolite concentrations were variable between individuals, and research into the causes of this variation should include factors such as breed, age, sex and neuter status to determine the impact of genetic and hormonal factors. Given the clinical importance of vitamin D in dogs, and the immense potential for utilising this species as a model for human disease, further elucidation of the vitamin D pathway in this species would provide immense clinical and research benefit.

Utilizing cooled liquid chromatography and chemical derivatization to separate and quantify C3-epimers of 25-hydroxy vitamin D and low abundant 1α,25(OH)2D3: Application in a pediatric population

There is need for a single assay able to quantify the most biologically active metabolite, 1α,25-dihydroxy-vitamin-D3, and the recently discovered biologically distinct C3-epimers of 25OHD, in addition to traditional vitamin D metabolites. We developed a method of chromatographic separation and absolute quantification of the following ten forms of vitamin D: 3-epi-25OHD3, 25OHD3, 3-epi-25OHD2, 25OHD2, 1α,25(OH)₂D3, 24R,25(OH)₂D3, 23R,25(OH)₂D3, 1a,25(OH)₂D2, D3, and D2 by single extraction and injection. Chemical derivatization followed by liquid chromatography using a charged surface hybrid C18 column and subsequent tandem mass spectrometry was utilized to detect and quantify each metabolite. This method is remarkable as a cooled column was required to achieve chromatographic resolution of epimers. Validation of each metabolite was performed at four concentrations and revealed inter- and intra-day precision and accuracy below 15% across three consecutive days of analysis. After validation, this method was applied to analyze the blood plasma from 739 samples from 352 subjects (8mo to 20 yr), 79 pooled plasma samples, and 10 NIST SRM972a samples. Healthy control samples (n = 357) were used to investigate developmentally associated changes in vitamin D metabolite concentrations during early life. This method yields excellent linearity (R² ≥ 0.99) across concentrations encompassing the biological range of many metabolites including 1α,25(OH)₂D3. Concentrations of 25OHD2 and 24R,25(OH)₂D3 were significantly (q ≤0.05) lower in infants compared to both children and adolescents. The percentage of 3-epi-25OHD3 in total 25OHD3 was significantly lower (q ≤ 0.009) in post-puberty subjects. Here we present a single assay capable of separating and quantifying ten vitamin D metabolites including C3-epimers of 25OHD, and quantifying 1α,25-dihydroxy-vitamin-D3 at and below concentrations observed in human plasma (LLOQ < 10 pM).

Impact of assay design on test performance: lessons learned from 25-hydroxyvitamin D

BACKGROUND

Current automated immunoassays vary significantly in many aspects of their design. This study sought to establish if the theoretical advantages and disadvantages associated with different design formats of automated 25-hydroxyvitamin D (25-OHD) assays are translated into variations in assay performance in practice.

METHODS

25-OHD was measured in 1236 samples using automated assays from Abbott, DiaSorin, Roche and Siemens. A subset of 362 samples had up to three liquid chromatography-tandem mass spectrometry 25-OHD analyses performed. 25-OHD₂ recovery, dilution recovery, human anti-animal antibody (HAAA) interference, 3-epi-25-OHD₃ cross-reactivity and precision of the automated assays were evaluated.

RESULTS

The assay that combined release of 25-OHD with analyte capture in a single step showed the most accurate 25-OHD₂ recovery and the best dilution recovery. The use of vitamin D binding protein (DBP) as the capture moiety was associated with 25-OHD₂ under-recovery, a trend consistent with 3-epi-25-OHD₃ cross-reactivity and immunity to HAAA interference. Assays using animal-derived antibodies did not show 3-epi-25-OHD₃ cross-reactivity but were variably susceptible to HAAA interference. Not combining 25-OHD release and capture in one step and use of biotin-streptavidin interaction for solid phase separation were features of the assays with inferior accuracy for diluted samples. The assays that used a backfill assay format showed the best precision at high concentrations but this design did not guarantee precision at low 25-OHD concentrations.

CONCLUSIONS

Variations in design among automated 25-OHD assays influence their performance characteristics. Consideration of the details of assay design is therefore important when selecting and validating new assays.

[Determination of vitamin D in calcium fortified foods using solid-phase extraction-high performuance liquid chromatography]

A method for the determination of vitamin D in calcium fortified foods using solid-phase extraction-high performance liquid chromatography has been developed. The Chromabond XTR solid-phase extraction column (14,500 mg, 70 mL) was used to extract and clean-up the sample. The calibration curve of vitamin D showed good linearity in the range of 0.1-100.0 microg/mL with correlation coefficient of 0.999. The limit of qualification was 0.01 microg/g and the limit of quantification was 0.03 microg/g. The average recoveries at three spiking levels were 106.2%, 99.5%, 100.1%, and the relative standard deviations were lower than 10%.

Vitamin D and its metabolites--supply of patients with various endocrine disorders and comparison of analytical methods

OBJECTIVE

Vitamin D is important not only for its effect on the homeostasis of calcium, but also for its anti-proliferative, pro-differentiation, pro-apoptotic and immuno-modulating effects in the tissues of living organisms. This work describes the results of serum 25(OH)D (n=2175) and 1alpha,25(OH)2D (n=2271) radioimmunoassays (RIA) conducted at the Institute of Endocrinology in Prague (Czech Republic) during the period of 2004-2006.

METHODS

Serum concentrations of vitamin D metabolites were determined by RIA kits from IDS Ltd., Boldon, UK. In a group of 20 healthy volunteers, results obtained using the RIA kit were compared with those obtained after serum extraction with acetonitrile, subsequent chromatographic (RP HPLC) separation of 25(OH)D and its detection in collected fractions using the same RIA kit from IDS.

RESULTS

The mean concentrations +/- S.D. in the samples studied were 76.4 +/- 45.6 nmol/l for 25(OH)D and 88.9 +/- 45.1 pmol/l for 1alpha,25(OH)2D. The mean concentrations of both metabolites were higher in women than in men, but only the difference for 1alpha,25(OH)2D) was statistically significant (p=0.0000). The lowest concentrations of 25(OH)D or 1alpha,25(OH)2D were found in patients with hyperparathyroidism, the highest concentrations those treated with cholecalciferol for osteoporosis and vitamin D deficiency. The relationship between 25(OH)D and 1alpha,25(OH)2D was expressed by the equation 1alpha,25(OH)2D = 71.0845 + 0.1890 * 25(OH)D (n=1065, p=0.0000, R2=0.0343). Based on the RIA results, only 4 % of individuals were inadequately supplied with 25(OH)D, 79 % were supplied adequately and almost 17 % were supplied more than adequately. Similarly, 1alpha,25(OH)2D shows inadequate concentration in 15 % of the individuals tested, 59 % of these individuals were within the reference range and 26 % had values exceeding the upper limit of the reference range. The mean concentrations of 25(OH)D obtained by HPLC separation were in average by 8 % higher than those obtained using only the RIA kit.

CONCLUSION

Interpretation of studies dealing with vitamin D stores must be based on precise and correct analytical procedures. However, only a few information exists about the influence of other vitamin D metabolites and conjugates on the concentration of immunoanalytically measured serum 25(OH)D or 1alpha,25(OH)2D.

Cestrum diurnum leaf as a source of 1,25(OH)2 Vitamin D3 improves egg shell thickness

A continuing concern of the poultry industry is the high incidence (12%) of egg losses in the laying house due to poor egg shell quality. Calcium (Ca) homeostasis is a key factor in egg shell formation. The economy of Ca utilisation is under the control of Vitamin D(3), particularly its active metabolite 1,25-dihydroxy cholecalciferol [1,25(OH)(2)D(3)]. Supplementation of 1,25(OH)(2)D(3) has been shown to increase specific gravity, shell thickness and shell weight of the egg. However, commercially available synthetic 1,25(OH)(2)D(3) is very expensive. Earlier studies from our Institute [Phytochemistry 37 (1994) 677] have identified a cheap, natural and rich source of 1,25(OH)(2)D(3) in the leaves of Cestrum diurnum (CD), a member of the Solanaceae family. In this study, CD leaves were explored as a source of 1,25(OH)(2)D(3) in the feed of layer birds to improve the egg shell thickness. Fifteen-week-old white leghorn layers were divided into four treatments of 60 birds each and as follows: (I) normal diet with Vitamin D(3), (II) normal diet with Vitamin D(3) + CD, (III) normal diet without Vitamin D(3) and, (IV) normal diet without Vitamin D(3) + CD powder. CD leaf powder was incorporated in to the feed at 0.3% level. The experimental feeding was continued up to 72 weeks of age of the birds. Weekly food intake and daily egg production were noted throughout the experimental period and the specific gravity of the eggs, feed consumed to lay one egg and egg shell thickness were determined. Incorporation of CD leaves in the feed had the maximal effect on all the parameters studied. The feed consumed to lay one egg was 20 g less than the control group. The specific gravity of the egg was higher by 0.005, than the control egg, indicating a 5% decrease in the breakage of eggs in CD fed chicks. Also there was a significant increase (P < 0.001) in egg shell thickness. The data suggest that incorporation of CD leaf powder in the feed of poultry layers increased the egg shell thickness, which in turn could decrease the economic loss due to breakage of eggs.

Rapid HPLC method for measurement of vitamin D3 and 25(OH)D3 in blood plasma

The present work describes a novel, simplified high-performance liquid chromatography (HPLC) method for evaluation of vitamin D3 and its 25-hydroxy metabolite in blood plasma. The retrieval of the analytes from the blood plasma matrix is based on a single-step extraction using acetonitrile. The method is specific, sensitive, and ensures good reproducibility. The recovery of the analytes, precision, and reproducibility obtained using the present approach gave results comparable to or better than more complex, laborious, and time-consuming procedures. This method is suitable for evaluation of the host's vitamin D physiological status, as well as for rapid analysis of blood plasma samples in suspected cholecalciferol toxicity. With a significantly shortened time of analysis (10 minutes), the present method allows the possibility for processing of a large number of samples rapidly, efficiently, and at a low cost.

Isolation, identification and biological activity of 24R,25-dihydroxy-3-epi-vitamin D3: a novel metabolite of 24R,25-dihydroxyvitamin D3 produced in rat osteosarcoma cells (UMR 106)

We recently identified 1alpha,25-dihydroxy-3-epi-vitamin D3 [1alpha,25(OH)2-3-epi-D3] as a metabolite of 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] produced in rat osteosarcoma cells (UMR 106). We now report the isolation of 24R,25-dihydroxy-3-epi-vitamin D3 [24R,25(OH)2-3-epi-D3] as a metabolite of 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3] by high-performance liquid chromatography (HPLC) with chiral column and its structure assignment by proton nuclear magnetic resonance (1H-NMR) and liquid chromatography-mass spectrometry (LC-MS) analysis. We also demonstrated the production of 24R,25(OH)2-3-epi-D, in two other cell lines [human colon carcinoma cells (Caco-2) and porcine kidney cells (LLC-PK1)] which were previously shown to convert 1alpha,25(OH)2D3 into 1alpha,25(OH)2-3-epi-D3. It can be seen that the production of 24R,25(OH)2- 3-epi-D3 from 24R,25(OH)2D3 is lower than that of 1alpha,25(OH)2-3-epi-D3 from 1alpha,25(OH)2D3 in all the cells studied. 24R,25(OH)2-3-epi-D3 was found to be inactive in terms of its ability to bind to the vitamin D receptor (VDR), in inhibiting proliferation and in inducing differentiation of human promyelocytic leukemia cells (HL-60). Thus, our study indicates that the C-3 epimerization pathway is common to both 1alpha,25(OH)2D3 and 24R,25(OH)2D3 and may play an important role in modulating the concentration and the biological activity of these two major vitamin D3 metabolites in target tissues.

Metabolism of 1alpha,25-dihydroxyvitamin D(3) and its C-3 epimer 1alpha,25-dihydroxy-3-epi-vitamin D(3) in neonatal human keratinocytes

We previously reported that 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] is metabolized into 1alpha,25-dihydroxy-3-epi-vitamin D(3) [1alpha,25(OH)(2)-3-epi-D(3)] in primary cultures of neonatal human keratinocytes. We now report that 1alpha,25(OH)(2)-3-epi-D(3) itself is further metabolized in human keratinocytes into several polar metabolites. One of the polar metabolite was unequivocally identified as 1alpha,23,25-trihydroxy-3-epi-vitamin D(3) by mass spectrometry and its sensitivity to sodium periodate. Three of the polar metabolites were identified as 1alpha,24,25-trihydroxy-3-epi-vitamin D(3), 1alpha,25-dihydroxy-24-oxo-3-epi-vitamin D(3) and 1alpha,23,25-trihydroxy-24-oxo-3-epi-vitamin D(3) by comigration with authentic standards on both straight and reverse phase HPLC systems. In addition to the polar metabolites, 1alpha,25(OH)(2)-3-epi-D(3) was also metabolized into two less polar metabolites. A possible structure of either 1alphaOH-3-epi-D(3)-20,25-cyclic ether or 1alphaOH-3-epi-D(3)-24,25-epoxide was assigned to one of the less polar metabolites through mass spectrometry. Thus, we indicate for the first time that 1alpha,25(OH)(2)-3-epi-D(3) is metabolized in neonatal human keratinocytes not only via the same C-24 and C-23 oxidation pathways like its parent, 1alpha,25(OH)(2)D(3); but also is metabolized into a less polar metabolite via a pathway that is unique to 1alpha,25(OH)(2)-3-epi-D(3).

Separation of vitamins by supercritical fluid chromatography with water-modified carbon dioxide as the mobile phase

Supercritical fluid chromatography (SFC) has become a technique for solving problems that are difficult to be monitored by other chromatographic methods. However, the most widely used fluid, is no more polar than hexane. Polar samples which are difficult to be analyzed with pure supercritical CO(2) because of their high polarity can be separated by adding polar modifiers to supercritical CO(2). In this paper various vitamins were well separated using water-modified supercritical CO(2) fluid. The amount of water dissolved in supercritical CO(2) was measured using an amperometric microsensor made of a thin film of perfluorosulfonate ionomer (PFSI).

A novel high-performance liquid chromatographic assay for vitamin D metabolites using a coulometric electrochemical detector

A new, highly sensitive HPLC assay method using an electrochemical detector (ECD) for multiple assay of vitamin D metabolites is reported. The assay involves extracting lipids from plasma with methylene chloride and methanol, purification on Zorbax SIL column with 5.5% (v/v) iso-propanol in hexane and quantification by HPLC-ECD. A coulometric system, composed of the dual electrode analytical cell and a guard cell, was used for ECD of the eluting compounds. The potentials applied to detectors 1 and 2 in a dual electrode analytical cell were adjusted to +0.20 V and +0.60 V, respectively. This method is sensitive to 20 pg of 25-hydroxyvitamin D3 [25(OH)D3] and of 24R,25-dihydroxyvitamin D3 [24,25(OH)2D3]. Calibration curves gave linearity from 20-1000 pg for 25(OH)D3 and 24,25(OH)2D3. The detection limit was approximately 50 pg ml-1 for 25(OH)D3 and 24,25(OH)2D3 in plasma. This sensitivity combined with an overall recovery of 25(OH)D3 (81.5 +/- 2.6%, mean +/- S.E.) allows the measurement of trace amount of 25(OH)D3 with only 20 microliters of plasma. Intra- and interassay RSD values were 5.3 and 9.7% for 25(OH)D3 and 6.3 and 9.7% for 24,25(OH)2D3, respectively. Plasma levels of 25(OH)D3 and 24,25(OH)2D3 in normal adults were 15.9 +/- 2.8 ng ml-1 (n = 10) and 1.4 +/- 0.5 ng ml-1 (n = 10), respectively. This method allows the determination of 25(OH)D2 and 25(OH)D3 for evaluating their nutritional and clinical status. From these results, it is concluded that the proposed HPLC-ECD assay system is useful for the determination of vitamin D metabolites in biological fluids as a highly sensitive physicochemical method.

[Extraction and purification of the three major vitamin D metabolites using C18 and NH2 cartridges and measurement of 25-hydroxyvitamin D]

In this study, the three major vitamin D metabolites: 25-hydroxyvitamin D [25(OH)D], 24,25-dihydroxy-vitamin D [24,25(OH)2D], 1,25-dihydroxyvitamin D [1,25(OH)2D] were clearly separated using a NH2 cartridge after acetonitrile and C18 cartridge extraction. In the NH2 cartridge purification procedure, 25(OH)D was eluted with hexane/dichloromethane (50:50), 24,25(OH)2D was eluted with hexane/dichloromethane (20:80) and 1,25(OH)2D was eluted with hexane/isopropanol (75:25). Contamination of each fraction with two other metabolites were less than 1.4%. Recoveries of added 3H-25(OH)D, 3H-24,25-(OH)2D and 3H-1,25(OH)2D were 73.2 +/- 2.45%, 60.0 +/- 2.98% and 63.5 +/- 3.37%, respectively. Using the 25(OH)D fraction after the NH2 cartridge procedure, we measured 25(OH)D using a competitive protein binding assay. The intra- (n = 10) and interassay (n = 8) coefficients of variation were 4.60-8.41% and 6.62-16.4%, respectively. Analytical recovery of added 25(OH)D was in the range of 81.2-130%. The 25(OH)D values were 17.4 +/- 6.02 ng/ml (mean +/- SD) in serum from 110 healthy volunteer collected in May. The correlation of 25(OH)D values was good between cartridge purification and high performance liquid chromatography (HPLC) purification. (gamma = -0.38 + 1.03x, r = 0.953, n = 36). This purification using a simple cartridge procedure was suitable for the measurement of 25(OH)D, and preferable to the time-consuming HPLC purification.

Retention behavior of conjugated metabolites of vitamin D and related compounds in high-performance liquid chromatography

The retention behavior of sulfates or glucuronides of provitamin D, vitamin D, and 25-hydroxyvitamin D3, together with its fluorescent derivatives, are examined with reversed-phase high-performance liquid chromatography. Inclusion chromatography using cyclodextrin as a mobile-phase additive is also used for this purpose. Conventional reversed-phase high-performance liquid chromatography clearly separates the positionally isomeric conjugates of 25-hydroxyvitamin D3. The addition of the host compound to the mobile phase is effective in separating the pairs of fluorescent derivatives of vitamin-D3 and -D2 conjugates or provitamin-D3 and -D2 conjugates.

Transfected human liver cytochrome P-450 hydroxylates vitamin D analogs at different side-chain positions

A full-length cDNA for the human liver mitochondrial cytochrome P-450 CYP27 was cloned from a human hepatoma HepG2 cDNA library and then subcloned into the mammalian expression vector pSG5. When CYP27 cDNA was transfected into COS-1 transformed monkey kidney cells along with adrenodoxin cDNA, transfected cells revealed a 10- to 20-fold higher vitamin D3-25-hydroxylase activity than nontransfected cells. Transfected cells were capable of 25-hydroxylation of vitamin D3, 1 alpha-hydroxyvitamin D3 and 1 alpha-hydroxydihydrotachysterol3. In each case they also showed the ability to 26(27)-hydroxylate the cholesterol-like (D3) side chain. The relative rates of 25- and 26(27)-hydroxylation of 1 alpha-hydroxyvitamin D3 approximately mimicked the ratio of products observed in HepG2 cells. Vitamin D2 and 1 alpha-hydroxyvitamin D2, both with the ergosterol-like side chain, were 24- and 26(27)-hydroxylated by CYP27. The rate of side-chain 24-, 25-, or 26(27)-hydroxylation was greater for 1 alpha-hydroxylated vitamin D analogs than for their nonhydroxylated counterparts. We conclude that CYP27 is capable of 24-, 25-, and 26(27)-hydroxylation of vitamin D analogs and that the nature of products is partially dictated by the side chain of the substrate. This work has revealed that the cytochrome P-450 CYP27 may be important in the metabolism of vitamin D analogs used as drugs.

The use of octadecyl-bonded microparticulate silica in the separation of free and bound fractions during saturation analysis of vitamin D metabolites

The use of octadecyl-bonded microparticulate silica to separate free and bound fractions during the saturation analysis of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D has been investigated. A slurry of octadecyl-bonded silica in an appropriate incubation buffer was prepared and used in parallel with a conventional dextran-coated charcoal suspension in several assay procedures. Standard curves, non-specific binding and plasma values were compared. A competitive protein binding assay for 25-hydroxyvitamin D and two radioreceptor assays and one radioimmunoassay for 1,25-dihydroxyvitamin D were investigated. In most cases the octadecyl-bonded silica preparation gave the more favourable results; its action was rapid, time- and temperature-independent, and it produced low non-specific binding and higher B0 values in all the assays examined. It was in our hands easier to use than dextran-coated charcoal. The use of octadecyl-bonded silica is recommended as an efficient agent for the separation of free and bound fractions in the saturation analysis of vitamin D metabolites.

[Determination of vitamin D in food using high performance liquid chromatography (HPLC). Results of collaborating studies of the working group "Vitamin Analysis" according to section 35 of the German Food Act]

A standardized method to determine the vitamin D content of food by means of HPLC is described. After the test material was homogenized and saponified with ethanolic aqueous potassium hydroxide solution, vitamin D was extracted using n-hexane. Using HPLC on a silica gel column, the fraction containing vitamin D is separated from the nonsaponifiable residue. After the fraction was reduced, the residue was dissolved in methanol and the vitamin D content determined after HPLC separation on an RP-C18 column. For evaluation, either a conventional external standard method using laboratory and matrix specific recovery rates as correction factors, or an internal standard method using vitamin D2, and D3, respectively as internal standards were employed. The method was developed and standardized by the working group "Vitamin Analysis" in accordance with section 35 LMBG (German Food Act). Repeatability and comparability of the results were checked in collaborative studies (14 laboratories) in milk powder and gruel that had been enriched with vitamin D3. Applicability of the method to other food (eggs, milk, fish, margarine) was checked separately. The statistical evaluation of the results of the collaborative studies has shown that the method is reliable enough to be included into the "Amtliche Sammlung" (official collection of analytical methods) according to section 35 LMBG. The present method may be used to determine the content of vitamin D2 and D3 in natural and vitaminized food. It is specific for the vitamins D2 and D3 released during saponification of food, but does not allow separate determination of the pre-vitamins D already present in the sample.

A simplified high-performance liquid chromatographic method for determination of vitamin D3, 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 in human serum

Measurement of the serum level of 25-hydroxyvitamin D is the most useful parameter in evaluating vitamin D status. The serum level of vitamin D is a useful parameter in studying short time effects after exposure to ultraviolet light and absorption of the vitamin after oral administration. A method for simultaneous determinations of vitamin D3 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 is described. Serum or plasma was extracted by methanol-isopropanol (90:10, v/v) and hexane. The hexane layer was injected in to a reversed-phase (C18) high-performance liquid chromatographic (HPLC) system. 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 were eluted by methanol-water (85:15, v/v), and vitamin D3 by a linear gradient of methanol-water (85:15) and methanol-isopropanol-water (87.5:10:2.5), and detected by u.v. absorption. This method gave separate determinations of the D2 and D3 forms of 25-hydroxyvitamin D, but owing to an interfering peak the method does not measure vitamin D2. The assay was very sensitive with a detection limit of 5 nmol l-1 for 25-hydroxyvitamin D2 and D3 and vitamin D3 by using 0.5 ml serum or plasma for analysis, so that for low vitamin D3 levels more than 1 ml of serum is desirable.

Induction of 25-OH-vitamin D3 24- and 23-hydroxylase activities in partially purified renal extracts from pigs given exogenous 1,25-(OH)2D3

A renal mitochondrial cytochrome P 450 preparation from pigs treated with exogenous 1,25-(OH)2D3 was reconstituted with an NADPH-generating system, adrenodoxin and adrenodoxin reductase. The reconstituted system catalyzed the conversion of the substrate, 25-OH-D3, to metabolites comigrating with authentic 23,25-(OH)2D3 and 24,25-(OH)2D3 in both straight- and reverse-phase high-performance liquid chromatography systems, which achieve separation of these metabolites from each other as well as from other vitamin D metabolites. The putative 23,25-(OH)2D3 product was resistant to periodate treatment, while the 24,25-(OH)2D3 product was sensitive, providing additional evidence for the identity of the products. Although induction of 24-hydroxylase activity has been studied using renal homogenates from several species, only recently have techniques become available to study the activity of the enzyme in a solubilized and reconstituted form. Using these techniques, the present study shows that production of 24,25-(OH)2D3 was increased more than 80-fold with 1,25-(OH)2D3 treatment compared with untreated controls, an effect much greater than that previously observed with homogenates. In addition, production of both 23,25-(OH)2D3 and 24,25-(OH)2D3 varied with substrate concentration and was consistent with a monooxygenase-linked enzyme reaction.

Tritium isotope effect in high-pressure liquid chromatography of vitamin D metabolites

A significant chromatographic isotope effect is reported for 1,25-dihydroxyvitamin D3 in a wide variety of HPLC separation systems. The effect is also observed for 24,25-dihydroxyvitamin D3. Retention times differ from less than 1% up to 4% depending on the separation system and the degree and position of tritium substitution. Such an effect must be corrected for whenever both labeled and unlabeled vitamin D metabolites are used in HPLC cochromatography or assay recovery studies.

Improved method for quantifying vitamin D in proprietary infants' formulas and in breast milk

In this new method for quantifying vitamin D in infants' formulas and breast milk, after repeated lipid extraction, samples are further purified by passage through Sep-Pak cartridges, followed by liquid chromatography, then quantified by competitive protein-binding assay. Analytical recovery is estimated by use of added 3H-labeled vitamin D. For repeated assays of a reconstituted proprietary powdered milk formula in two runs, the intra-assay vitamin D values were 9.0 (SD 1.4) (n = 6) and 8.9 (SD 1.6) micrograms/L (n = 7) (t = 0.07). Assay of a proprietary liquid formula yielded values of 14.9 (SD 0.9) micrograms/L (n = 6). For each, the results agreed with the vitamin D content shown on the label. Vitamin D concentrations in breast milk from two groups of lactating mothers of different social class and nutritional status were 2.3 (SD 1.4) (n = 7) and 2.0 (SD 1.7) micrograms/L (n = 7). Overall recoveries ranged from 65 to 75%. Only 2 mL of milk is required, which facilitates sample collection, and the assay is less time-consuming than other current methods.

Solid phase extraction system for vitamin D and its major metabolites in human plasma

A new procedure using C18 and silica cartridges for the extraction and subsequent separation of vitamin D and its major metabolites from plasma has been developed and compared to a conventional extraction procedure with respect to lipophilic material extracted as evaluated by high-performance liquid chromatographic profiles. The C18 cartridges were efficient in extracting all compounds tested while subsequent chromatography of the extract on silica cartridges was effective in resolving vitamin D and its metabolites based on increasing polarity. High-performance liquid chromatographic profiles of each silica cartridge fraction clearly demonstrated that the newly conceived solid phase extraction was superior to conventional extraction methods with respect to cleanliness of sample fractions. This difference in lipophilic load between the new and conventional extraction systems was most apparent in the vitamin D and 25-hydroxyvitamin D containing fractions. The new extraction system can be used when total extraction and subsequent analysis of vitamin D and its major metabolites is desired.

19-nor-10-ketovitamin D derivatives: unique metabolites of vitamin D3, vitamin D2, and 25-hydroxyvitamin D3

Three metabolites were isolated after incubation of vitamin D3, vitamin D2, or 25-hydroxyvitamin D3 with bovine rumen microbes. They are identified as 5(E)-19-nor-10-ketovitamin D3, 5(E)-19-nor-10-ketovitamin D2, and 5(E)-19-nor-10-keto-25-hydroxyvitamin D3, respectively. The identifications were based on ultraviolet absorbance, mass spectroscopy, and chemical reactivity. All 5(E)-19-nor-10-ketovitamin D derivatives examined had an absorbance maximum at 312 nm and a characteristic fragment in their mass spectra corresponding to loss of 43 amu from their molecular ions. The vitamin D3 metabolite was identical in all essential spectral and chromatographic aspects with authentic synthetic 5(E)-19-nor-10-ketovitamin D3. These metabolites represent a unique pathway of vitamin D metabolism and the first characterized products of microbial vitamin D metabolism. The conversion of vitamin D and its metabolites to their 19-nor-10-keto forms likely represents a detoxification mechanism.

Vitamin D and its metabolites in human and bovine milk

Human and bovine milk were analyzed for vitamin D, 25-hydroxyvitamin D, 24,25-dihydroxyvitamin D, 25,26-dihydroxyvitamin D and 1,25-dihydroxyvitamin D using exhaustive chromatographic purification procedures coupled with ligand binding assays. Human milk contained the following amounts of antirachitic sterols (pg/ml, mean +/- SD, n = 5): 39 +/- 9 vitamin D; 311 +/- 31 25-hydroxyvitamin D; 52 +/- 8 24,25-hydroxyvitamin D; 32 +/- 9 25,26-dihydroxyvitamin D; 5.1 +/- 0.3 1,25-dihydroxyvitamin D. Normal bovine milk contained levels of these sterols comparable to those found in human milk. Increasing the oral dose of vitamin D to the cows was reflected by an increase of the parent vitamin and 25-hydroxyvitamin D in the milk. Vitamin D-binding protein concentration in human milk whey, determined by Ouchterlony immunodiffusion and radioimmunoassay, was 1--2% of the levels observed in the plasma and was dependent on the stage of lactation. Vitamin D and its metabolites were shown initially to be present in the whey portion but with time migrated into the fat portion of milk. The antirachitic sterols detected account for approximately 25 IU/liter and 27 IU/liter of antirachitic activity in human and bovine milk, respectively. In both species 25-hydroxyvitamin D comprised the majority of the antirachitic sterols detected in normal milk.

The isolation and characterization of the 25-hydroxyvitamin D-binding protein from chick serum

The binding protein for 15-hydroxyvitamin D3 (25-OH-D3) was isolated from chick serum. The molecular weight was 60,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, without the existence of sub-units. Its sedimentation coefficient was 4.3 S. The isoelectric points of the major and minor components were 6.1 and 5.9, respectively. Addition of 25-hydroxyvitamin D3 causes an anodal shift in the isoelectric point of the two forms of the binding protein. Heating at 60 degrees C slowly destroys the binding protein but the addition of vitamin D metabolites increases its thermostability. The binding protein has probably a single binding site for all vitamin D metabolites and its association constant for 25-OH-D3 at 4 degrees C and pH 7.4 is 10(9) M-1. The same affinity is found for 24,25-dihydroxyvitamin D3 but 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 have a 3- and 10-fold lower affinity, respectively. The amino acid composition is very similar to that of the human and rat binding protein. The concentration of chick 25-hydroxyvitamin D3-binding protein was higher in egg-laying hens (10 microM) than in immature hens (4 microM) or immature or adult roosters (4 microM). Chick plasma contains virtually only a 4 S form of the 25-hydroxyvitamin D3-binding protein but partially hemolyzed chick serum also contains a 6 S form. Addition of pure actin to chick plasma or serum converts all 4 S form to the 6 S form. The serum 25-hydroxyvitamin D3-binding protein is thus heterogenous since it exists in two 4 S forms (with different pI) and a 6 S form (complexed with actin). No evidence was found for the existence of a separate vitamin D3-binding protein.

Isolation and partial characterization of two immunologically similar vitamin D-binding proteins in rat serum

Two immunologically similar, probably identical, binding proteins for vitamin D and its metabolites (DBP1 and DBP2) were isolated separately from rat serum after approximately 180-fold purification by novel procedures using Blue Sepharose CL-6B chromatography. The freshly purified DBP1 and DBP2 each showed a single band of protein on polyacrylamide gel electrophoresis, and had alpha-mobility, although DBP1 moved slightly faster than DBP2. DBP1 and DBP2 had the same molecular weight, which was estimated as approximately 54,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The isoelectric points of DBP1 and DBP2 were estimated as 4.9 and 5.0, respectively, from the results of isoelectric focusing experiments. DBP1 and DBP2 both appeared to have one binding site for 25-hydroxyvitamin D3 per molecule of protein, with apparently similar association constants at 4 degrees C of 5--7 x 10(9) M-1. The amino acid compositions of DBP1 and DBP2 were also determined and compared. A monospecific antiserum against rat DBP2 was prepared in a rabbit and was used for immunological studies of rat DBP. On double immunodiffusion, anti-DBP2 antiserum produced precipitin lines of complete reaction-of-identity against the purified DBP1, the purified DBP2, and rat whole serum. There was no immunological cross-reactivity between rat DBP and sera from man, dog, and rabbit, but mouse serum showed a pattern of partial identity with rat DBP. When rat serum samples were analyzed by immunoelectrophoresis using anti-DBP antiserum, three patterns of precipitin line were observed: a pattern showing the existence of only DBP1, designated as DBP 1-1; a pattern showing the existence of only DBP2, designated as DBP 2-2; and a pattern showing the existence of both DBP1 and DBP2, designated as DBP 2-1. Using single radial immunodiffusion assay for rat serum DBP, the mean (+/- S.D.) serum DBP concentrations were found to be 461 +/- 59 microgram/ml in adult male rats and 328 +/- 16 microgram/ml in adult female rats, and the difference was significant (p < 0.001). In molar terms, DBPs are present in normal rat serum in large excess relative to vitamin D and its metabolites, and most of the serum DBP, therefore, circulates as apo-DBP, not containing a bound molecule of vitamin D or of its metabolites. The immunoprecipitation studies of DBP in rat serum showed that DBPs were common main transport proteins for naturally occurring vitamin D and its metabolites, and that DBP played some, but not a principal, role in the transport of synthetic 1 alpha-hydroxyvitamin D3.

Quantitation of the main metabolites of vitamin D in a single serum sample. II. Determination by UV-absorption and competitive protein binding assays

The present report, together with extraction, separation and purification procedures described previously [1], constitutes a sensitive method for determination of vitamin D and its main metabolites in 2 ml of human serum. By omitting vitamin D itself, 25-OHD, 24,25-(OH)2D, 25,26-(OH)2D and 1,25-(OH)2D could be determined in less than 0.5 ml serum. The quantitation methods described have detection limits of 1.8 pmol for vitamin D, 20 fmol for 25-OHD, 24,25-(OH)2D and 25,26-(OH)2D, and 1 fmol for 1,25-(OH)2D.

Quantitation of the main metabolites of vitamin D in a single serum sample. I. Extraction, separation and purification of metabolites

A method for extraction, separation and purification of the main serum metabolites of vitamin D from a single serum sample is described. The method involved extraction of serum by diethylether and separation and purification of vitamin D, 25-OHD and the dihydroxymetabolites 24,25-(OH)2D, 25,26-(OH),2D and 1,25-(OH)2D by elution in three steps from a short open silicic acid column. The eluted vitamin D metabolites were further separated and purified by high pressure liquid chromatography (HPLC). The HPLC systems described separated the D2 and D3 forms of vitamin D, 25-OHD, 1,25-(OH)2D, and probably also 24,25-(OH)2D and 25,26-(OH)2D. The metabolites were purified by the methods described for further quantitation by UV-absorption or competitive protein binding assays, and were found to be homogenous on re-chromatography with different HPLC systems. Good recoveries were obtained for all the metabolites.

The transport of vitamin D and its 25-hydroxy metabolite in human plasma. Isolation and partial characterization of vitamin D and 25-hydroxyvitamin D binding protein

This study reports the isolation and partial characterization of vitamin D and 25-hydroxyvitamin D binding protein (DBP), the specific transport protein for vitamin D and its 25-hydroxy metabolite in human plasma. DBP was labeled by the addition of a tracer amount of 3H-labeled 25-OH-D3 to the original plasma used for protein fractionation. Previous experiments have shown that such 25-OH-D3 added in vitro binds to the same protein normally responsible for the transport of endogenous 25-OH-D and of vitamin D. The isolation of human DBP was achieved by an extensive sequence of procedures which resulted in a final yield of only approximately 4 mg of purified DBP from a starting volume of 34 liters of plasma. Purified DBP was homogeneous in the analytical ultracentrifuge and showed a single band of protein on analytical polyacrylamide gel electrophoresis. DBP had a sedimentation constant of 3.49s and a mol wt of approximately 52,000. The molecular weight was assessed by sedimentation equilibrium analysis and also by sodium dodecyl sulfate-disc-gel electrophoresis and by gel filtration on a standardized column of Sephadex G-150. The amino acid composition of DBP was determined and was generally consistent with the estimated extinction coefficient (E1cm1% at 280 nm) of about 9.1. The isoelectric point of DBP was estimated as 4.8 from isoelectric focusing experiments. Direct study of the binding capacity of the purified DBP for added 25-OH-D3 showed that the isolated DBP had a high affinity for 25-OH-D3, with an apparent maximum binding capacity of one molecule of 25-OH-D3 per molecule of protein.